In the field of liquid compositions for treating the surface of tin-plated DI can of the aforementioned type, one example is the invention disclosed in (1) Japanese Patent Application Laid Open [Kokai or Unexamined] Number 01-100,281 [100,281/89]. This particular invention comprises a film-forming liquid composition for the treatment of metal surfaces in which the liquid composition has a pH of 2 to 6 and contains 1 to 50 g/L of phosphate ions, 0.2 to 20.0 g/L of oxyacid ions, 0.01 to 5.0 g/L of tin ions, and 0.01 to 5.0 g/L of condensed phosphate ion. Treatment with this conversion treatment liquid composition lays down a strongly corrosion-resistant phosphate film on the surface of a tin-plated DI can.
The inventions disclosed in (2) Japanese Patent Application Laid Open Number 01-172,406 [172,406,/89] and U. S. Pat. No. 4,457,790 are examples of a treatment method intended to develop corrosion resistance and adhesiveness through the use of water soluble resin. These inventions teach methods for treating metal surfaces with a solution which contains a derivative of a suitable polyhydric phenol compound and an aminomethylene substituted phenolic polymer or oligomer respectively.
The treatment baths previously employed to treat the surface of aluminum and aluminum alloy of the aforementioned type can be generally classified into chromate types and non-chromate types. The chromate types typically take the form of chromic acid chromate conversion treatments and phosphoric acid chromate conversion treatments. Chromic acid chromate conversion treatments entered into practical application in about 1950, and these are still widely used for heat exchanger fin material and the like. This type of conversion treatment bath is based on chromic acid (CrO.sub.3) and hydrofluoric acid (HF) and also contains an accelerator. It lays down a film which contains modest quantities of hexavalent chromium.
The phosphoric acid chromate conversion treatment derives from the invention in U.S. Pat. Number 2,438,877 (1945), and this type of conversion treatment bath is based on chromic acid (CrO.sub.3), phosphoric acid (H.sub.3 PO.sub.4), and hydrofluoric acid (HF). It forms a film whose principal component is hydrated chromium phosphate (CrPO.sub.4.4H.sub.2 O). As this film does not contain hexavalent chromium, it is currently widely employed as a paint undercoating treatment for the body and lid of beverage cans.
The invention disclosed in Japanese Patent Application Laid Open [Kokai or Unexamined] Number 52-131937 [131,937/77] is a typical prior art example within the realm of non-chromate types of treatments for aluminum. The disclosed treatment bath comprises an acidic aqueous coating solution (pH=approximately 1.0 to 4.0) which contains zirconium or titanium or a mixture thereof, as well as phosphate and fluoride. The application of this conversion treatment bath exemplary of the prior art to the surface of aluminum generates a conversion film whose principal component is zirconium oxide or titanium oxide. While the absence of hexavalent chromium is an advantage associated with non-chromate type treatment baths, the corrosion resistance and paint adherence achieved with such baths in the prior art are inferior to those with chromate type treatments. Furthermore, both the chromate type treatments and non-chromate type treatments contain fluorine, while environmental considerations have recently created demand for a fluorine-free surface treatment bath.
Within the sphere of treatment methods which use water-soluble resin in order to impart corrosion resistance and paint adherence to aluminum, the following are listed as exemplary of the prior art: Japanese Patent Application Laid Open Number 61-91369 [91,369/86] and Japanese Patent Application Laid Open Number 01-172406 [172,406/89]. In these methods, the metal surface is treated with a solution which contains a useable derivative of a polyhydric phenolic compound. However, it is difficult with these methods to form an adequately stable film on the surface of aluminum, and this precludes the appearance of a satisfactory performance (corrosion resistance).
During the metal can manufacturing process, the high friction coefficient of the exterior can surface causes the can surface to have a poor slideability during conveyor transport of the can, which causes the can to tumble ever sideways and thus impairs the transport operation. Can transportability is a particular issue with respect to transport to the printer in a high speed continuous manufacturing plant. It is therefore important in the can manufacturing industry to reduce the static friction coefficient of the exterior can surface without compromising the adhesiveness of any paint or lacquer to be coated on the can. The invention disclosed in (3) Japanese Patent Application Laid Open Number 64-85292 [85,292/89] is an example of a method for improving the slidability. This invention concerns an agent for treating the surfaces of metal cans. This particular agent contains water-soluble organic material selected from phosphate esters, alcohols, monovalent and polyvalent fatty acids, fatty acid derivatives, and mixtures of the preceding.